1. Field of the Invention
This invention involves optical fiber which supports essentially only a single guided mode, perhaps degenerate, at the transmission wavelength, usually between 0.6 and 1.7 microns. The index of refraction of the core material is graded in the radial direction so as to yield an optical fiber with very low total dispersion and therefore high bandwidth.
2. Description of the Prior Art
Since the work of Maxwell in the late nineteenth century, elucidating the electromagnetic wave nature of light, it has been known that electromagnetic radiation in the optical region of the spectrum is inherently capable of transmitting more information than electromagnetic radiation in lower frequency (higher wavelength) regions of the spectrum. Clearly, optical radiation, for the purposes of this application referring generally to electromagnetic radiation in the region of the spectrum between 0.6 and 1.7 microns, is likewise capable of carrying more information than electrical impulses transmitted through copper wire. Despite the many years since the recognition of the information carrying capabilities of light, commercial optical communication systems have not been developed, due primarily to lack of a sufficiently low loss transmission medium.
In the mid-1960's, it was suggested by Kao and Hockham (Proceedings of IEE, Vol. 113, No. 7, July 1966, page 1151) that pure silica would be able to transmit light in the optical region of the electromagnetic spectrum with losses less than 20 dB/km, a value viewed by many as representing the onset of commercial, economic viability. Hence silica would constitute the long searched-for low loss optical transmission medium.
In the early 1970's, a number of manufacturing techniques were developed to form silica based fibers of loss less than 20 dB/km. Such processes depended, by and large, upon the formation of silica-based glass from appropriate glass precursor vapors. Processes include the Bell System MCVD process (U.S. Pat. No. 4,217,027), the Corning soot process (U.S. Pat. Nos. 3,711,262, Re. 28,029), and the VAD process (U.S. Pat. Nos. 3,966,446, 4,135,901, 4,224,046) pursued, among others, by a number of Japanese companies. As a result of these developments, optical fibers are now routinely fabricated in commercial processes with losses less than 2 dB/km in the optical region of the spectrum.
While losses associated with optical fibers have been reduced, such reduction is due primarily to the selection of appropriate material systems and to the use of appropriate fabrication processes. Added losses due to cabling, or induced by curvature, microbending, or splicing, continue to be of some concern, especially when ultra-low loss fibers (less than 1 dB/km) are considered.
Despite the continuing concern with fiber loss, fiber characteristics have reached the point where repeater spacing is in many instances limited, not by the loss characteristics of the fiber, but rather by the bandwidth characteristics of the fiber. Repeaters are required where the optical signal is well above detectable levels, since the individual pulses have reached a point where they overlap sufficiently to result in bandwidth degradation.
To understand the bandwidth limitation of optical fibers it should be remembered that, originally, fibers were constructed primarily with parameters appropriate to the support of numerous propagating modes within a single fiber, hence the term "multimode fibers." Different propagation velocities, associated with each of these modes, result in dispersion, commonly referred to as mode dispersion, which affects the bandwidth of the fiber deleteriously. Early in the development of optical fibers, it was appreciated radial gradations in the index of refraction resulted in lowered mode dispersion and improved bandwidth of multi-mode fibers.
Despite the increased bandwidth obtained when multi-mode fibers are appropriately graded in index of refraction in the radial direction, bandwidths in excess of 500 megahertz-kilometer could not be readily obtained using commercially viable fabrication processes. This failure to obtain ultra-high bandwidth using multi-mode fiber led to a concentration of research effort on single-mode fibers, which, it was known, could theoretically have bandwidths in the gigahertz-kilometer region. Although such high bandwidths had been predicted early in the theoretical study of optical fibers, commercially viable fabrication processes had not been developed to realize this, as yet theoretical, high bandwidth.
Initial fabrication of single-mode fibers involved a step index structure in which the core was primarily of a uniform index of refraction and the cladding was primarily of a uniform lower index of refraction. Early fibers comprised silica cores with downdoped claddings of, for example, borosilicate and later fluorosilicate. Later fibers included undoped cores of, for example, germania silicate and silica claddings. However, both of these designs were impractical from a manufacturing standpoint due to the high temperatures necessary to process deposited pure silica. Distortion in the fiber due to these high temperatures in the manufacturing process led away from the use of pure silica in step index single-mode fibers. Later fibers included germania silicate cores and phosphosilicate claddings. Phosphorus in the cladding simplifies the manufacturing process insofar as it lowers the melting temperature of silica and acts as a fining agent. Furthermore, the removal of boron from the fiber, whose presence likewise simplifies manufacturing due to lowered melting temperatures, avoids the relatively low wavelength infrared absorption edge associated with borosilicate glasses.
Recent developments have centered about the manufacture of ever higher bandwidth single-mode fibers. The pursuit of such low dispersion designs has led to the realization that material dispersion was not the only significant consideration in designing high bandwidth fibers, but waveguide dispersion must likewise be taken in to account. Waveguide dispersion is associated with the specific field configurations of the electromagnetic radiation within the fiber. Field configurations, for example, which result in significant transmission of power near the core-clad interface will have different dispersive characteristics than field configurations which result in significant transmission of power near the fiber core. Waveguide dispersion varies with the physical characteristics of the fiber, such as core radius, and with material characteristics such as .DELTA., the relative index difference, .alpha. the index profile, N the absolute index value and with .lambda. the transmission wavelength. For given fiber parameters there are regions of the spectrum where the waveguide dispersion is of opposite sign that the material dispersion. Therefore fibers may be designed so that the waveguide and material dispersion will cancel, yielding essentially zero dispersion along a narrow range of wavelengths (W. A. Gambling et al., Electronics Letters, Vol. 15, No. 15, July 19, 1979, page 475).
The high bandwidth, zero total dispersion characteristics which are obtained by offsetting material dispersion against waveguide dispersion and which yield desirably high information transmission rates, may be obtained over a relatively wide spectral range by appropriate selection of index profile in both core and cladding. Fibers of the future, which may include frequency multiplexing, namely, the transmission of information at a number of different frequencies simultaneously, must be designed with high bandwidths at all of the frequencies envisioned for future transmission. Appropriate index profiles which yield bandwidth characteristics that meet this need include "w type" fibers. (See, for example, S. Kawakami et al., Proceedings of IEE, Vol. 123, No. 6, June 1976, page 586.) Such fibers, sometimes referred to as doubly clad single-mode fibers, include a core region adjacent to a first cladding region of index of refraction lower than the core, surrounded by a second cladding region with an index of refraction intermediate between the core and the first cladding region. It is found that such w type fibers may be designed with low dispersion over a relatively extended range of wavelengths.
While the above discussion of single-mode fibers has been in terms of the step index design, it was early realized that due to manufacturing limitations, it was rarely, if ever, possible to produce an ideal step-index single-mode fiber. Early studies showed, for example, that inhomogeneity in the index of refraction of the core, due to diffusion effects which could not be totally avoided during manufacture, resulted in shifted cutoff frequencies, i.e., that frequency below which only a single mode is transmitted in the core. (See, for example, W. A. Gambling et al., Electronics Letters, Vol. 13, No. 5, Mar. 3, 1977, page 139.) Later studies concentrated on the effects of intentionally graded single-mode fibers. Such work additionally studied the effect of the relatively prevalent central dip in the index of refraction obtained in certain manufacturing processes. These studies indicated that fibers with an index profile parameter .alpha..ltoreq.2 could not be designed with zero dispersion, and furthermore showed that the central dip only exacerbated this problem. (See, for example, A. W. Snyder et al., Electronics Letters, Vol. 15, No. 10, May 10, 1979, page 269.) Other studies showed that for .alpha..ltoreq.2, zero dispersion could be obtained, but that the contribution to the dispersive characteristics of the fiber associated with waveguide dispersion was essentially zero, and that the zero dispersion point consequently was determined by the zero for material dispersion alone.